Chemical and biological separations are routinely performed in various industrial and academic settings to determine the presence and/or quantity of individual species in complex sample mixtures. There exist various techniques for performing such separations. One separation technique, liquid chromatography, encompasses a number of methods that are used for separating closely related components of mixtures. Liquid chromatography is a physical method of separation involving a sample (or sample extract) being dissolved in a mobile phase (which may be a gas, a liquid, or a supercritical fluid). While carrying the sample, the mobile phase is then forced (e.g., by gravity, by applying pressure, or by applying an electric field) through a separation ‘column’ containing an immobile, immiscible stationary phase. Liquid chromatography has many applications including separation, identification, purification, and quantification of compounds within various mixtures.
One category of conventional chromatography includes pressure-driven systems. These systems are operated by supplying a pressurized mobile phase (typically one or more liquid solvents pressurized with a pump) to a separation column. Typical columns have dimensions of several (e.g., 10, 15, 25) centimeters in length and between 3–5 millimeters in diameter, with capillary columns typically having internal diameters between 3–200 microns. The columns are packed with particulate-based stationary phase material typically consisting of very small diameter (e.g., 5 or 10 micron) particles. It is important to minimize any voids in a packed column, since voids or other irregularities in a separation system can affect the quality of the results of the separation. Thus, most conventional separation columns include specially designed end fittings (typically having compressible ferrule regions) designed to hold packed stationary phase material in place and prevent irregular flow-through regions.
Important components of conventional chromatography columns are fine porous materials, commonly referred to as “frits,” that retains the stationary phase material within the columns as separations are performed. Each end of a column typically includes a frit. Frits for conventional high performance liquid chromatography (HPLC) columns are typically composed of either a metal, such as stainless steel or titanium, or a polymer, such as polyethylene (PE) or poly (ether ether ketone) (PEEK). The pore sizes for frits used with five-micron stationary phase particles are typically about two microns. The thickness of such frits typically is between about thirty mils (about 760 microns) and about seventy-five mils (about 2000 microns).
There has been a growing interest in the manufacture and use of microfluidic systems to perform chromatography. This is because, when conducted in microfluidic volumes, chromatography may be carried out using very small volumes of liquid that enhance safety and reduce disposal quantities. One difficulty in fabricating microfluidic devices having integral HPLC columns, however, has been including frits within such devices.
Novel methods for fabricating microfluidic separation devices are disclosed in commonly-assigned U.S. Patent Application Publication No. 2003/0150806. A plurality of stacked device layers or sheets define microfluidic structures within the device that form multiple separation columns. The columns are defined in one or more of the device layers by cutting or otherwise removing portions of the device layer such that the remaining portions of the device layer form the lateral boundaries or “walls” of the microstructures. The microstructures are completed by sandwiching a wall-defining device layer between substrates and/or other device layers to form the “floors” and “ceilings” of the microstructures. The use of multi-layer construction permits robust devices to be fabricated quickly and inexpensively compared to surface micromachining or material deposition techniques that are conventionally employed to produce microfluidic devices.
FIGS. 1A–1C show a simplified multi-layer microfluidic separation device 10 having a plurality of separation columns 22A–22E defined therein (with numbering for columns 22B–22D omitted for clarity). It will be readily understood by one skilled in the art that the device 10 illustrated in FIGS. 1A–1B has been simplified to illustrate the basic structure associated with multi-layer microfluidic separation devices and are not intended to limit the scope of the invention. Referring to FIG. 1C, the device 10 is fabricated with at least four device layers 14–17. The second device layer 16 defines the lateral boundaries of a plurality of separation columns 22A–22E. The third device layer 15 defines the lateral boundaries of a plurality of exit channels 24A–24E. The first and third device layers 14, 16 define the lower and upper boundaries, respectively, of the exit channels 24A–24E and the second and fourth device layers 15, 17 define the lower and upper boundaries, respectively, of the separation columns 22A–22E. A stationary phase material 20 is retained in the separation columns 22A–22E by a frit 26 positioned between the second and third device layers 15, 16. Thus, mobile phase solvent (as well as the sample compound being separated) flows through the system as indicated by arrows 30, while the stationary phase material 20 is kept in place by the frit 26.
Frit materials used with conventional chromatography columns have thicknesses typically ranging between about thirty mils (760 microns) and about seventy-five mils (2000 microns). Because multi-layer multifluidic devices typically use device layer materials having thicknesses ranging from about one mil (twenty-five microns) to about twenty-five mils (635 microns), conventional HPLC frit materials are too thick to be used within a laminated multi-layer microfluidic separation device.
Moreover, certain conventional frit materials, such as stainless steel, may be difficult to bond to the polymer layers of a stacked layer device. In fact, conventional polymer frit materials also may be difficult to bond to other polymers, particularly where it is desirable to avoid the use of adhesives that could contaminate a microfluidic device. Adhesiveless bonding techniques may be used (e.g., by applying heat, pressure or a combination thereof) to attempt to bond a frit material directly to the surrounding device layers. However, it has been found that when frits are composed of material closely related to the material of the surrounding device layers, the temperature required to achieve the desired bonding tends to melt the frit to the degree that it is rendered inoperable or its effectiveness is reduced. When dissimilar materials are used, with desirable melting point differentials, device layer materials and frit materials bond less effectively, frequently resulting in undesirable separation of the layers from the frits at operational pressures.
Initial efforts to incorporate thin polymeric frit materials in multi-layer microfluidic separation devices included materials such as Nuclepore™, a track-etched polycarbonate membrane having a thickness of 6–11 microns, a pore size of 0.015–12.0 microns, and a pore density of 1×105–6×108 pores/cm2 (Whatman, Inc., Clifton, N.J.) (the “polycarbonate frit”). The polycarbonate frit presented several issues related to the fabrication and operation of the assembled device. First, the pore size of the polycarbonate frit is larger than the size of the stationary phase particulate material. Devices made with the polycarbonate frit suffered from a lack of reproducible pressure drop, which was believed to be caused by the clogging or blocking of the pores of the frit with the stationary phase material.
In addition, the polycarbonate frits have significantly different surface energy than the polymeric films most desirable for use in fabricating microfluidic separation devices (particularly polyolefins, including polypropylene). Also, the polycarbonate frits have at least one, if not two, very smooth surfaces. As a result, the bonding between the polycarbonate frit and the surrounding polymeric film was relatively weak, occasionally permitting undesirable fluid flow around the frit. For example, track-etched polycarbonate frits were selected to prevent wicking within the frit itself (such as that shown in FIG. 1B and indicated by flow arrow 28). However, because of the significantly different energies between the polycarbonate frit and the device layers and/or the smoothness of the polycarbonate frit, the bond therebetween was weak and allowed lateral wicking across the surface of the frit (such as that shown in FIG. 1B and indicated by flow arrow 29), thereby permitting cross contamination between separation columns 20D, 20E. Thus, to avoid wicking, as shown in FIG. 2, a device 100 using a polycarbonate frit would require multiple discrete frits 126A–126E, each placed in contact with one of the five separation channels 122A–122E, adding complexity and time to the assembly process for such devices.
However, even when multiple discrete frits are used, there may be some wicking of fluid between each discrete frit and the surrounding device layers. Thus, while the use of discrete frits prevents cross-contamination between adjacent columns, fluid may be retained in the frit region during a first chromatographic separation only to contaminate subsequent chromatographic separations in the same column.
Thus, it would be desirable to provide a frit material that is very thin, minimizes or eliminates wicking, and may be readily bonded to the device layers of a laminated multi-layer microfluidic device. It also would be desirable to provide a frit material that bonds sufficiently to the surrounding device layers to minimize or eliminate fluid retention in the frit region.